The present invention relates to the use of quantum dots in light emitting diodes. The invention further relates to a light emitting diode which emits light of a tailored spectrum of frequencies.
Light emitting diodes (LEDs) are ubiquitous to modern display technology. More than 30 billion chips are produced each year and new applications, such as automobile lights and traffic signals, continue to grow. Conventional diodes are made from inorganic compound semiconductors, typically AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue). These diodes emit monochromatic light of a frequency corresponding to the band gap of the compound semiconductor used in the device. Thus, conventional LEDs cannot emit white light, or indeed, light of any xe2x80x9cmixedxe2x80x9d color, which is composed of a mixture of frequencies. Further, producing an LED even of a particular desired xe2x80x9cpurexe2x80x9d single-frequency color can be difficult, since excellent control of semiconductor chemistry is required.
LEDs of mixed colors, and particularly white LEDs, have many potential applications. Consumers would prefer white light in many displays currently having red or green LEDs. White LEDs could be used as light sources with existing color filter technology to produce fall color displays. Moreover, the use of white LEDs could lead to lower cost and simpler fabrication than red-green-blue LED technology. There is currently one technology for producing white LEDs, which combines a blue LED with a yellow phosphor to produce white light. However, color control is poor with this technology, since the colors of the LED and the phosphor cannot be varied. This technology also cannot be used to produce light of other mixed colors.
It has also been proposed to manufacture white or colored LEDs by combining various derivatives of photoluminescent polymers such as poly(phenylene vinylene) (PPVs). One device which has been proposed involves a PPV coating over a blue GaN LED, where the light from the LED stimulates emission in the characteristic color of the PPV, so that the observed light is composed of a mixture of the characteristic colors of the LED and the PPV. However, the maximum theoretical quantum yield for PPV-based devices is 25%, and the color control is often poor, since organic materials tend to fluoresce in rather wide spectra. Furthermore, PPVs are rather difficult to manufacture reliably, since they are degraded by light, oxygen, and water. Related approaches use blue GaN-based LEDs coated with a thin film of organic dyes, but efficiencies are low (see, for example, Guha, et al., J Appl. Phys. 82(8):4126-4128, Oct. 1997; III-Vs Review 10(l):4, 1997).
It has also been proposed to produce LEDs of varying colors by the use of quantum dots. Semiconductor nanocrystallites (quantum dots) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of quantum dots shift to the blue (higher energies) as the size of the dots gets smaller. It has been found that a CdSe quantum dot, for example, can emit light in any monochromatic, visible color, where the particular color characteristic of that dot is dependent only on its size.
Currently available light-emitting diodes and related devices which incorporate quantum dots use dots which have been grown epitaxially on a semiconductor layer. This fabrication technique is suitable for the production of infra-red LEDs, but LEDs in higher-energy colors have not been achieved by this method. Further, the processing costs of epitaxial growth by currently available methods (molecular beam epitaxy and chemical vapor deposition) are quite high. Colloidal production of dots is a much more inexpensive process, but these dots have generally been found to exhibit low quantum efficiencies, and thus have not previously been considered suitable for incorporation into light-emitting diodes.
A few proposals have been made for embedding colloidally produced quantum dots in an electrically conductive layer, in order to use the electroluminescence of these dots for an LED, but such devices require a transparent, electrically conductive host matrix, which severely limits the available materials for producing LEDs by this method. Available host matrix materials are often themselves light-emitting, which may limit the achievable colors using this method.
In one aspect, this invention comprises an electronic device, comprising a solid-state light source, and a population of quantum dots disposed in a host matrix. The quantum dots are characterized by a band gap smaller than the energy of at least a portion of the light from the light source. The matrix is disposed in a configuration that allows light from the source to pass therethrough. When the host matrix is irradiated by light from the source, that light causes the quantum dots to photoluminesce secondary light. The color of this light is a function of the size of the quantum dots.
In one embodiment of this aspect, the quantum dots comprise CdS, CdSe, CdTe, ZnS, or ZnSe and may optionally be overcoated with a material comprising ZnS, ZnSe, CdS, CdSe, CdTe, or MgSe. The quantum dots may be further coated with a material having an affinity for the host matrix. The host matrix may be a polymer such as polystyrene, polyimide, or epoxy, a silica glass, or a silica gel. The primary light source may be a light-emitting diode, a solid-state laser, or a solid-state ultraviolet source. The color of the device is determined by the size distribution of the quantum dots; this distribution may exhibit one or more narrow peaks. The quantum dots, for example, may be selected to have no more than a 10% rms deviation in the size of the dots. The light may be of a pure color, or a mixed color, including pure white.
In a related aspect, the invention comprises a method of producing an electronic device as described above. In this method, a population of quantum dots is provided, and these dots are dispersed in a host matrix. A solid-state light source is then provided to illuminate the dots, thereby causing them to photoluminesce light of a color characteristic of their size distribution. The dots may be colloidally produced (i.e., by precipitation and/or growth from solution), and may comprise CdS, CdSe, CdTe, ZnS, or ZnSe. They may further comprise an overcoat comprising ZnS, ZnSe, CdS, CdSe, CdTe, or MgSe. The host matrix may be any material in which quantum dots may be dispersed in a configuration in which they may be illuminated by the primary light source. Some examples of host matrix materials are polymers such as polystyrene, polyimide, or epoxy, silica glasses, or silica gels. Any solid-state light source capable of causing the quantum dots to photoluminesce may be used; some examples are light-emitting diodes, solid-state lasers, and solid-state ultraviolet sources.
It may be desirable to tailor the size distribution of the quantum dots in order to tailor the color of light which is produced by the device. In one embodiment, the dots exhibit no more than a 10% rms deviation in diameter. The light may be of a pure color (corresponding to a monodisperse size distribution of quantum dots), or a mixed color (corresponding to a polydisperse size distribution of quantum dots) including white.
In a further aspect, the invention comprises a quantum dot colloid, in which quantum dots are disposed in a nonconductive host matrix. The quantum dots may be coated with a material having an affinity for the host matrix. When illuminated by a primary source of light of a higher energy than the band gap energy of the dots, the quantum dots photoluminesce in a color characteristic of their size distribution.
In one embodiment, the dots comprise CdS, CdSe, CdTe, ZnS, or ZnSe, optionally overcoated with a material comprising ZnS, ZnSe, CdS, CdSe, CdTe, or MgSe. The nonconductive host matrix may be a polymer such as polystyrene, polyimide, or epoxy, a silica glass, or a silica gel. In one embodiment, the dots are coated with a monomer related to a polymer component of the host matrix. The dots may be selected to have a size distribution exhibiting an rms deviation in diameter of less than 10%; this embodiment will cause the dots to photoluminesce in a pure color.
A related aspect of the invention comprises a prepolymer colloid. In this aspect, the invention comprises a liquid or semisolid precursor material, with a population of quantum dots disposed therein. The colloid is capable of being reacted, for example by polymerization, to form a solid, transparent, nonconductive host matrix. The quantum dots may have been coated with a material having an affinity for the precursor material. The precursor material may be a monomer, which can be reacted to form a polymer. The quantum dots may comprise CdS, CdSe, CdTe, ZnS, or ZnSe, and may optionally be overcoated with a material comprising ZnS, ZnSe, CdS, CdSe, CdTe, or MgSe. The dots may be selected to have a size distribution having an rms deviation in diameter of less than 10%.
In yet another aspect, the invention comprises a method of producing light of a selected color. The method comprises the steps of providing a population of quantum dots disposed in a host matrix, and irradiating the host matrix with a solid-state source of light having an energy high enough to cause the quantum dots to photoluminesce. The quantum dots may comprise CdS, CdSe, CdTe, ZnS, or ZnSe, and may further have an overcoating comprising ZnS, ZnSe, CdS, CdSe, CdTe, or MgSe. The host matrix may comprise polymers such as polystyrene, polyimide, or epoxy, silica glasses, or silica gels.
The host matrix containing the quantum dots may be formed by reacting a precursor material having quantum dots disposed therein (for example by polymerization). Alternatively, two or more precursor materials may be provided, each having a different size distribution of quantum dots disposed therein. These precursors may be mixed and reacted to form a host matrix, or alternatively, they may be layered to form a host matrix having different size distributions of quantum dots in different layers.
As used herein, the phrase xe2x80x9ccolloidally grownxe2x80x9d quantum dots refers to dots which have been produced by precipitation and/or growth from a solution. A distinction between these dots and quantum dots epitaxially grown on a substrate is that colloidally grown dots have a substantially uniform surface energy, while epitaxially grown dots usually have different surface energies on the face in contact with the substrate and on the remainder of the dot surface.
As used herein, the terms xe2x80x9cpurexe2x80x9d or xe2x80x9cmonochromaticxe2x80x9d color refers to a color which is composed of light of a single frequency. A xe2x80x9cmixedxe2x80x9d or xe2x80x9cpolychromaticxe2x80x9d color refers to a color which is composed of light of a mixture of different frequencies.
As used herein, a xe2x80x9cmonomerxe2x80x9d is a substance which can be polymerized according to techniques known in the art of materials science, and may include oligomers. A xe2x80x9crelated monomerxe2x80x9d of a polymer is a component monomer of the polymer, or a compound capable of being incorporated into the backbone of the polymer chain.